This invention relates to an optical sensor which delivers signals representing errors of focusing of an optical system on a surface. These signals are employed for controlling the lens-to-image distance of the optical system which is thus made dependent on correct focusing.
This invention applies more particularly to optical reading of information on a movable recording medium and especially a videodisk.
For these applications, the depth of field of the optical focusing system is of the order of one micron and it is therefore essential to correct errors relating to the lens-to-image distance of the optical system and caused by the motion of the recording medium.
As disclosed in U.S. Pat. No. 4,079,247, it is a known practice to utilize part of the optical beam reflected from the recording medium for the purpose of obtaining an astigmatic beam by means of a cylindrical lens after the reflected beam has passed through the focusing lens. Four juxtaposed cells forming a square are placed in the plane corresponding to the circle of least confusion in which a circular light spot is present if focusing is correct. In the event of defocusing, the spot undergoes deformation which is detected by the signals of the four cells. There is thus obtained an error signal which is amplified and produces action on the control motor for adjusting the lens-to-image distance.
In accordance with the cited patent, the reading signal can also be, the obtained from the signals delivered by the cells.